Gas turbines usually burn hydrocarbon fuels and produce air polluting emissions such as oxides of nitrogen (NOx) and carbon monoxide (CO). Oxidization of molecular nitrogen in the gas turbine depends upon the temperature of gas located in a combustor, as well as the residence time for reactants located in the highest temperature regions within the combustor. Thus, the amount of NOx produced by the gas turbine may be reduced by either maintaining the combustor temperature below a temperature at which NOx is produced, or by limiting the residence time of the reactant in the combustor.
One approach for controlling the temperature of the combustor involves pre-mixing fuel and air to create a lean fuel-air mixture prior to combustion. This approach may include the axial staging of fuel injection where a first fuel-air mixture is injected and ignited at a first or primary combustion zone of the combustor to produce a main flow of high energy combustion gases, and where a second fuel-air mixture is injected into and mixed with the main flow of high energy combustion gases via a plurality of radially oriented and circumferentially spaced fuel injectors or axially staged fuel injectors positioned downstream from the primary combustion zone. Axially staged injection increases the likelihood of complete combustion of available fuel, which in turn reduces the air polluting emissions.
During operation of the combustor, it is necessary to cool one or more liners or ducts that form a combustion chamber and/or a hot gas path through the combustor. Liner cooling is typically achieved by routing a cooling medium such as the compressed air through a cooling flow annulus or flow passage defined between the liner and a flow sleeve and/or an impingement sleeve that surrounds the liner. However, in particular configurations, one or more bluff bodies such the axially staged fuel injectors or mounting hardware such as a mounting boss for the axially staged fuel injectors are disposed within the cooling flow annulus, thereby disrupting the cooling flow through the cooling flow annulus. Each bluff body creates a wake region just behind or downstream therefrom, thereby reducing overall cooling effectiveness of the cooling medium, particularly in the wake region.